#include "apue.h"
#include <errno.h>
#include <fcntl.h>
#include <sys/wait.h>

int main(int argc, char *argv[]) {
    int fd;
    pid_t pid;
    char buf[5];
    struct stat statbuf;
    if (argc != 2)
        err_sys("usage: %s filename\n", argv[0]);

    if ((fd= open(argv[1], O_RDWR | O_CREAT | O_TRUNC, FILE_MODE)) < 0)
        err_sys("open error");
    if (write(fd, "abcdef", 6) < 0)
        err_sys("write error");

    if ((fstat(fd, &statbuf)) < 0)
        err_sys("fstat error");
    if (fchmod(fd, (statbuf.st_mode & ~S_IXGRP) | S_ISGID) < 0)
        err_sys("fchmod error");

    TELL_WAIT();
    if ((pid= fork()) < 0)
        err_sys("fork error");
    if (pid > 0) { /* parent */
        if (write_lock(fd, 0, SEEK_SET, 0) < 0)
            err_sys("write_lock error");
        TELL_CHILD(pid);
        if (waitpid(pid, NULL, 0) < 0)
            err_sys("waitpid error");

    } else {    /*child */
        WAIT_PARENT();
        set_fl(fd, O_NONBLOCK);
        /*父进程已获得写锁，所以这里只可能返回 -1 */
        if (read_lock(fd, 0, SEEK_SET, 0) != -1)    /* no wait */
            err_sys("child: read_lock succeeded");
        printf("read_lock of already-locked region returns %d\n", errno);

        if (lseek(fd, 0, SEEK_SET) == -1)
            err_sys("lseek error");
        if (read(fd, buf, 2) < 0)
            err_ret("read failed (mandatory locking works)");
        else 
            printf("read OK (no mandatory locking), buf = %2.2s\n", buf);
    }
    exit(0);
}

